1. Field of the Invention
The present invention relates to a speed detecting apparatus to be used for detecting the moving speed of an object, more particularly to an optical pickup which is moved radially relative to a record disc.
2. Description of the Prior Art
Generally, in an optical record reproducing apparatus such as a compact disc player, it is well known for a pickup drive apparatus with a linear motor as a driving source to be used for moving an optical pickup radially relative to an optical disc. In such a compact disc player, a speed detecting apparatus for detecting the moving speed of the optical pickup is provided corresponding to the pickup drive apparatus and high-accuracy positioning of the optical pickup is performed by controlling the linear motor in conjunction with the moving speed signal detected by the speed detecting apparatus.
FIG. 9 shows a conventional speed detecting apparatus together with a pickup device and a pickup drive apparatus.
Numeral 1 designates an optical pickup main body, the upper face of which faces an optical disc (not shown in the drawing). The body 1 is mounted on guide shafts 3 and 4 and is movable in the direction of the arrows A and B on bearings 2 (only one of them is shown in the drawing). On the upper surface portion of the optical pickup main body 1, there is a focusing lens 1a for irradiating a laser beam and, on the under surface portion, a support plate 5 is provided. At one end portion of the support plate 5 there is mounted a linear motor 6 for radial drive. The linear motor 6 comprises a coil unit, for example, a bobbin 6b wound with a drive coil 6a, attached to the support plate 5, and a box-shaped magnetic member 6, or yoke, one side of which passes through the bobbin 6b. A permanent magnet 6d is attached to the inside of the magnetic member 6c, facing the drive coil 6a, to form a magnetic field.
On the other end of the support plate 5, there is attached a coil unit 7 for detecting the optical pickup moving speed. For example, the coil unit 7 may include a bobbin 7a wound with a detecting coil 7b (not shown). One side of a frame-shaped magnetic member or yoke 8 passes freely through the coil unit 7. The frame-shaped magnetic member 8 may be constructed from two or four separate pieces. A magnet 9 is attached to the inside the magnetic member 8, facing the detecting coil 7b, to form a magnetic field.
In the construction described above, the drive coil, 6a is provided in a magnetic circuit, formed by the magnetic member 6c and the magnet 6d and, when a current is applied to the drive coil 6a, an energizing force conforming to Fleming's left hand rule is generated. The optical pickup main body 1 is thus moved in the direction of arrows A and B through the bobbin 6b, according to the direction of the current, and a laser beam is radiated on to the optical disc (not shown in the drawing) through the focusing lens 1a. Simultaneously, the support plate 5 with attached bobbin 7a wound with the detecting coil 7b moves in the same direction. Since the detecting coil 7b is provided in a magnetic field formed with the magnetic member 8 and the magnet 9 and cuts the magnetic flux therebetween, an inductive electromotive force is generated in the coil in proportion to the moving speed, conforming to Fleming's so-called right hand rule, and a detecting section (not shown in the drawing) detects the moving speed of the optical pickup main body 1 from the changes in the electromotive force.
Through this operation, the detecting section (not shown in the drawing) controls application of the current to the drive coil 6a of the linear motor 6 and controls the position of the optical pickup main body 1.
However, in such a speed detecting apparatus there is the possibility that an external electric field can have an adverse influence upon the performance of the device. For example, when the linear motor 6 is used as described above, it is necessary to space it from the position of the speed detecting apparatus so that the apparatus is not detrimentally influenced by the magnetic field. As a result, the combined structure must be made larger.
Also, there is some clearance produced between the bearing 2 and the guide shaft 3 of the optical pickup main body 1. When the drive coil 6a is moved at a high speed in the direction of the arrows A and B, the drive section of the optical pickup main body 1 is rotated by the inertial force of the drive system around the axis of inertia positioned between the drive coil 6a and the detecting coil 7b. As a result, the drive coil 6a and the detecting coil 7b have speed components (speed vectors) in opposite directions from one another. The phase of the control signal fed back from the detecting coil 7b to the drive coil 6 is reversed and the drive system oscillates resulting in degradation of control.
The same problems arise in the case of a signal treatment apparatus with an optical pickup device utilized in an acoustic instrument such as a laser disc and in the case of an optical pickup and a magnetic head utilized in an information apparatus such as an optical disc memory apparatus or a magnetic disc memory apparatus.